The present invention relates generally to cooling techniques for optical elements employed in laser systems, and more particularly, to conductive face-cooled optical elements for use in laser systems.
The process of frequency conversion in a nonlinear laser material generates heat within the nonlinear material medium due to absorption. This heat must be removed if the frequency converter is to operate efficiently at a significant power level. Also, the process of storing energy in a solid state laser amplifier material also generates heat within the laser medium that must be removed, especially if the amplifier is to operate at a significant input power.
A traditional method of heat removal in solid state crystalline materials employed in laser systems is to remove the heat from the sides of the materials, in a direction transverse to the direction of laser energy propagation. The removal of heat in a transverse direction causes thermal gradients in this direction. This creates two problems. The first problem is that thermal-optical stress and index variations cause thermal aberrations that distort the laser beam. The second problem is that, in most frequency conversion materials, for example, the temperature variation in a direction transverse to the direction of propagation of the laser beam must be maintained to within a very small tolerance range. The presence of a thermal gradient in this direction severely limits the aperture size and the power loading allowed in a laser system design. Transverse cooling is described in a paper entitled "The Potential of High-Average-Power Solid State Lasers," by J. L. Emmett et al., Document No. UCRL-53571, dated Sep. 25, 1984, available from the National Technical Information Service.
Conventional beam shaping techniques have been used to cool crystals whereby the laser beam is optically flattened in one direction. This allows the crystal to be cooled along a greater length, and reduces the path from the center of the beam to the edge of the crystal where it is cooled. However, this method is not practical in all applications, and mates a relatively high degree of complexity in the associated optics.
In some crystalline materials, and in particular .beta.-barium borate (BBO), the direction of greatest thermal conductivity in the material is also aligned closely with direction of optical propagation. In order to efficiently remove heat from materials with this property, the heat must therefore be removed from the optical faces. One method of face cooling is a convective process, normally achieved using a flowing gas. In this method, a gas is forced at high velocity across the faces of the crystal. The chief disadvantage of this method is that it requires a complex, active cooling system, and is therefore less suitable for applications requiring low cost, weight and volume, and a high degree of reliability. Also the engineering is complex because the gas flow across the optical surfaces must be very uniform to avoid optical distortion.
Therefore, it would be an advance in the art to have a heat removal technique that removes heat from the optical faces of a laser crystal or other optical element in a direction parallel to the beam path, and does it in a completely passive way.